(1) Field of the Invention
This invention relates to a demodulating circuit for a data signal. More specifically, the present invention relates to a circuit for demodulating a data signal from a modulated signal burst, in which a carrier signal is moduled by a data signal, that is, from a signal of the type in which the periods during which a modulated signal is present is intermittent.
(2) Description of the Prior Art
Data signal transmission systems include a system which transmits modulated signal bursts prepared by modulating a carrier signal by pulse signals of "1", "0", etc., representing data, or rather a system which transmits such a modulated signal only when a data signal is present. When a data signal from such a transmission system is received and demodulated, it is well known to prepare a reference carrier signal in synchronism with the carrier, and demodulate the data signal using a phase comparator or the like. To ensure that the demodulator operates stably, time periods during which there is no data signal, only the carrier signal, are provided before and after the time period of the data signal.
When such a modulated signal burst is demodulated by the demodulator, noise occurs in the periods during which there is no carrier signal. For this reason, a carrier sense circuit is usually provided parallel to the demodulator to detect the period containing the carrier, and an output from the carrier sense circuit enables output from the demodulator only during the period in which the carrier is present.
However, in practice, the operation of this carrier sense circuit is subject to a certain time delay. In other words, the output of the carrier sense circuit lags slightly behind the period of time in which the carrier signal is present. This results in the problem that the period during which noise is generated in the demodulator's output does not exactly coincide with the period during which the output from the carrier sense circuit, that is, a signal range indicating the absence of the carrier, is off. Therefore, this circuit can not completely eliminate noise from the output of the demodulator.
This noise signal has an adverse effect upon any equipment (e.g. a computer) to which the data signal which is the output of the demodulator is input.